<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
    <style>
        *,*::before,*::after{
            margin: 0;
            padding: 0;
            box-sizing: border-box;
            /* 
                移动端常用布局是非固定宽度
             */
             -webkit-box-sizing: border-box;
        }
        div{
            height: 100px;
            width: 100px;
            border: 20px solid;
            width: 100%;
        }
    </style>
<body>
    <div>

    </div>
</body>
</html>